1. Field of the Invention
This invention relates to multiphoton absorption of infrared radiation and, more particularly, to enhancing multiphoton absorption of infrared radiation by electronic excitation of polyatomic molecules.
2. Description of the Prior Art
Among the advances in photochemistry made possible by the laser is "multiphoton absorption," which refers to the absorption by a polyatomic molecule of a large number of photons--generally infrared--of the same energy. In absorbing the photons, the molecule is excited up the ladder of vibrational levels to a highly excited vibrational state. Since the energy separations between adjacent vibrational levels are not equal, a series of photons of different energies corresponding to the successive level separations would appear to be necessary. However, in intense IR laser fields, a very large number of IR photons all having the same energy may be absorbed by a polyatomic molecule, leading to collisionless photodissociation or predissociation or to other chemical reactions.
Multiphoton infrared laser excitation has been discussed extensively in the scientific literature (see, e.g., A. S. Sudbo, P. A. Schulz, Y. R. Shen and Y. T. Lee, J. Chem. Phys. 69, 2312 (1978) and R. V. Ambartzumian and V. S. Letokhov in Chemical and Biochemical Applications of Lasers, Vol. III, edited by C. B. Moore (Academic Press, New York, 1977)) and has been used as an isotope separation method (see, e.g., U.S. Pat. No. 4,049,515, issued Sept. 20, 1977 to Robinson et al. and U.S. Pat. No. 3,937,956, issued Feb. 10, 1976 to Lyon).
A disadvantage of prior art multiphoton excitation is the high fluence (product of intensity and time) required for excitation. This high fluence threshold, generally greater than about 10 J/cm.sup.2, requires the use of costly, high-power infrared lasers. High fluence requirements also limit operating pressure, since for fluence of .about.50 J/cm.sup.2, dielectric breakdown will generally occur, unless pressure is .ltorsim.1 kPa. Low operating pressure implies low production rate.
Karny et al., Chem. Phys., 37, 15 (1979), have reported multiphoton absorption and corresponding induced electronic emission from chromium oxychloride (CrO.sub.2 Cl.sub.2) following excitation by a focused 1 J CO.sub.2 laser tuned to R(30) at 10.2 .mu.m. Since prior art multiphoton absorption requires that excitation be very close to resonance, they found no evidence of absorption when irradiation was in the adjacent P branch at 10.6 .mu.m or in the 9.6 .mu.m band.